Fig. 2: Phase transformations of BM-SFO → SrFeO_2.75 → P-SFO observed on BM-SFO/STO with starting OVCs parallel to STO(001).

a–c Selected in-situ TEM images (see Supplementary Movie 1) of the same region of a ≈35 nm SFO film illustrating stages of the phase transition process over ≈300 s: the initial state of BM-SFO (a), appearance of the P-SFO inclusions (b), and propagation of the P-SFO phase (from the bottom of the image) (c). d–f High resolution TEM images and g–i, Fast Fourier Transform patterns (bottom left) and simulated diffraction patterns (bottom right) for (d) BM-SFO, (e) SrFeO_2.75, and (f) P-SFO. The scale bars are 2 nm⁻¹. j Structure model of bulk-like SrFeO_2.75. k Schematics showing the creation of oxygen vacancies (V_Os) in SFO/STO by annealing in vacuum, and subsequent replenishment of oxygen by annealing in ¹⁸O₂. l ToF-SIMS depth profiles of ¹⁸O display the enrichment level for a 15 nm SFO film grown on STO(001) and a STO reference sample. The SFO/STO interface location, marked by the black dashed line, was confirmed by the secondary ion signals of FeO^- and TiO^-.